1. Field of the Invention
The present invention relates to lasers. More specifically, the present invention relates to pumplight driven solid-state laser amplifiers that employ rectangular slab geometry.
2. Description of the Related Art
Solid-state lasers employ one or more amplifier stages to achieve a desired output power. Solid-state laser amplifier designs employ a doped-insulator lasing medium that may be driven by high power light emitting diode array pumplights, but can also employ flashlamps or other pumplight technology. The lasing material is typically comprised of a host crystal doped with an ion, such as, for example, ytterbium doped yttrium aluminum garnet (Yb:YAG) or neodymium doped yttrium aluminum garnet (Nd:YAG). Modern slab laser amplifiers are optically pumped by narrow spectral band, high brightness laser diode arrays. The higher brightness levels of such laser diode pump sources allow the high aspect ratio slab to be pumped either through the narrow edges of the slab, in directions generally transverse to the laser beam, or through the narrow ends of the slab in directions generally co-linear with the laser beam. Edge and end pumping of the slab allows the broad side faces to be cooled without constraining the cooling system to also transmit the pumplight beam into the slab, thereby generally simplifying the cooling system design by not requiring the coolant to transmit the pumplight beam. Laser efficiency is improved with a pumplight configuration that results in the optimum absorption and distribution of pumplight energy in the lasing medium.
In each amplifier stage of a laser system, a laser beam from an oscillator or previous amplifier state is directed into the entrance end of a gain medium slab. The driving laser beam is directed through the slab, and a lasing action occurs when the dopant ions release energy to the beam as they revert to their previous stable-low-energy state. A portion of the energy released is converted to light and results in an energy gain in the laser beam traversing the slab. Such lasers operate in both pulsed, and CW modes of operation.
The physical operating characteristics of a solid-state laser present many challenges to designers. Naturally, the process is not 100% efficient; so all designs must deal with residual energy issues. For example, not all of the pumplight energy is coupled to the dopant ions, which results in waste energy in the form of sensible heat and fluorescent energy. Further, not all of the energy delivered to the dopant ions is recovered by the laser beam as actual laser gain. Waste energy that is not radiated out of the slab as fluorescent energy must be conducted out of the slab as sensible heat energy. Slab surface cooling techniques are employed to conduct heat out of the slab, but this results in significant thermal gradients within the slab.
Thermal gradients within the slab are problematic. Thermal gradients result in crystal stress gradients that are known to produce stress induced birefringence. Birefringence results in beam depolarization. Fortunately, the rectangular slab geometry enables designers to maintain an orthogonal relationship between the stress gradient and the beam polarization, which greatly controls the effect of beam depolarization in the presence of slab stress. Another significant problem is thermal lensing, which is caused by the temperature gradient within the slab. Essentially, the slab's index of refraction is temperature dependent, so a gradient of temperatures within the slab results in a gradient of refraction angles of the beam as it passes through the slab. Rectangular slab solid-state lasers employ a ‘tophat’ beam that has a size and aspect ratio approximately the same shape and size as the entry end of the gain medium. Thermal lensing causes the beam shape to change as it propagates so that is does not continue to conform to the cross section of the gain medium though the slab. The net effect is that the beam is diffused, and sometimes focused, as the various light rays pass through different temperature regions within the slab. Beam quality is compromised. Further, the beam tends to over fill or under fill the slab. As a result, optical distortions are produced, beam quality is degraded, and general system performance is reduced as well.
The thermal lensing in the slab is cylindrical and can be compensated by propagating the beam in a zigzag path between the broad slab surfaces. The zigzag path causes all of the light rays to pass through all of the temperature gradients within the slab, netting out the thermal lens effect. However, a zigzag path through the slab also defines certain shadow zones where the laser beam does not reach dopant ions that have been elevated to a metastable state by the pumplight energy. This results in both reduced laser gain extraction efficiency and increased waste energy that must be dealt with. Multiple passes of the laser beam through the slab have been used in the prior art to more completely fill the slab, but the extended path that the laser beam must traverse results in unwanted beam expansion, reduced beam quality, and a generally more complex system design. Beam expansion implies that the entrance end beam aspect ratio must either be initially under filled to allow for the beam expansion as it propagates, or that the expanded beam will be clipped and produce stray rays that result in excessive internal reflections and feedback. Stray energy and feedback reduce beam quality.
Thus there is a need in the art for a laser amplifier apparatus and method that provides more complete fill of the slab gain medium, increases available power extraction, simplifies system design, and improves systems efficiency and performance.